1. Field of the Invention
The present invention relates to a semiconductor memory device, more particularly to control circuits included in a semiconductor memory device which is preferably fabricated as a programmable read only memory (PROM).
2. Description of the Prior Art
Various types of semiconductor memory devices (hereinafter also called simply memories) have been put into practical use. The present invention mainly refers to a PROM. In a usual PROM, a bit selection, word selection, and read operation are achieved under a power source voltage of V.sub.cc. During a write operation thereof, however, a program control voltage V.sub.p is needed which exceeds the level of V.sub.cc, for example, 20 V.
A PROM is usually supplied for commercial sale in a standard one-chip package. Therefore, the number of input/output pins is limited. Because of this, a pin may often have to be commonly used as a terminal for two or more different input/output signals. Such common use of a pin poses no problem if the input/output signals have the same voltage levels. However, there is a problem if a pin is commonly used as an input terminal for both a program control voltage V.sub.p and a usual input control signal S.sub.c having a voltage of V.sub.sc.
In such a case, a switching circuit must be employed so as to prevent malfunctions in the constant current source which drives the program circuits connected to the common use pin. That is, the switching circuit operates to transfer the desired program control voltage V.sub.p to the constant current source and the program circuits selectively, only when the program control voltage V.sub.p is applied to the pin. The present invention specifically relates to such a switching circuit.
There is, however, a problem with the memory in that undesired channel leakage in the transistor comprising the constant current source often occurs, resulting in erroneous operation.